In many ink printheads, the aperture plate and jet stacks typically consist of stainless steel plates. The aperture plate has an array of small holes, or nozzles, also sometimes referred to as jets, through which the ink exits the jet stack. Stainless steel aperture plates and other plates in the jet stack are being replaced with flexible, polymer layers such as polyimide. In some instances, the polyimide film receives an anti-wetting coating, is bonded to a stainless steel aperture plate, and then a laser ablates the array of apertures into the polyimide film.
Drooling apertures or nozzles, wetting and adhesion of ink on the printhead front face lead to missing and misdirectional jetting along with poor image quality. Drooling nozzles weep ink when the internal pressure of the printhead exceeds a particular pressure, typically measured in inches of water. The higher pressure the nozzles can maintain without weeping leads to higher jetting latitude and improved performance. Wetting occurs when the front face of the printhead remains wet after printing. This ink that remains on the printhead can block the nozzles resulting in missing nozzles and misdirectional printing. FIG. 1 shows a photograph of such a contaminated printhead.
Currently, one approach to overcome these issues is to use an active cleaning blade system. Typically, these wiper blades are used only in two circumstances: when missing jets are detected and after a power-down when the ink has frozen, shrunk, and drawn air into the system. The system purges ink from the printhead and a wiper blade then wipes the ink off the front face. The ink purge expels contamination, trapped air and clears the nozzles, and then the wipers clean off the front face. With an expected printhead lifetime of 6 years, daily purges will require roughly 2000 purge and wipe cycles. This increase in wipes means that any anti-wetting coating will have to survive and maintain their beneficial properties for over 2000 cycles.
The anti-wetting coating must also have a high contact angle to maintain adequate drool pressure and a low slide angle to maintain an easy clean/self clean feature. This will lead to printhead cartridges with low or no maintenance, high engine reliability and low run cost. The stack manufacturing process generally involves high temperatures and pressures, so the coating must maintain these properties under these conditions, typically 290° C. and 350 psi for approximately 30 minutes. Generally low-adhesion anti-wetting coatings having a low slide angle have shown to slide off the printhead front face cleanly under gravity.
Oleophobic, low adhesion surface coatings for solid inks have provided significant performance improvements. Oleophobic low adhesion anti-wetting coatings disposed on polyimide aperture plate prototypes have been shown to enable high drool pressure and self/easy clean feature with solid inks.
However, while such coatings have been beneficial for use with solid inks, developing low adhesion anti-wetting surface coatings for aqueous inks is a particularly challenging task. Most aqueous inks have a complicated chemistry and include components like surfactants. Due to the surfactants, aqueous inks tend to wet most surfaces. In fact, even hydrophobic, high water contact angle surfaces are wetted by aqueous inks. Wetting printhead aperture plate surfaces leads to drooling or low drool pressure resulting in failure. Also, printhead maintenance requirements result in a higher and longer ink-anti-wetting coating contact than in solid inks e.g. due to capping of heads during off times to prevent ink drying. To date, only a very few polymers can achieve a robust aqueous ink anti-wetting coating. Hence, the development of further robust anti-wetting coatings for aqueous inks is needed.